In many electronic applications, electrical resonators are used. For example, in many wireless communications devices, radio frequency (rf) and microwave frequency resonators are used as filters to improve reception and transmission of signals. Filters typically include inductors and capacitors, and more recently resonators.
As will be appreciated, it is desirable to reduce the size of components of electronic devices. Many known filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves are converted into electrical waves for use in electrical applications.
Acoustic resonators convert electrical signals to acoustic signals (sound waves) and convert received acoustic waves to electrical signals via inverse and direct piezoelectric effect. Acoustic transducers generally include acoustic resonators, such as surface acoustic wave (SAW) resonators and bulk acoustic wave (BAW) resonators, and may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, BAW resonators include layer bulk acoustic resonators (FBARs), which include resonator stacks formed over a substrate cavity, and solidly mounted resonators (SMRs), which include resonator stacks formed over an acoustic reflector (e.g., Bragg mirror). The BAW resonators may be used for electrical filters and voltage transformers, for example.
Generally, an acoustic resonator has a layer of piezoelectric material between two conductive plates (electrodes), which may be formed on a thin membrane. The piezoelectric material may be a layer of various materials, such as aluminum nitride (AlN), zinc oxide (ZnO), or lead zirconate titanate (PZT), for example.
One of the indicators of a characteristic of the piezoelectric layer is an electromechanical coupling coefficient (kt2). The electromechanical coupling coefficient is a coefficient indicating the efficiency of conversion between electric energy and mechanical vibration energy. In a frequency filter using a piezoelectric layer, the larger the electromechanical coupling coefficient of a piezoelectric material is, the wider the frequency is the bandwidth of the filter.
While various methods to increase the bandwidth of a filter comprising acoustic resonators have been investigated, there is an ongoing need to improve the bandwidth by increasing the electromechanical coupling coefficient of the piezoelectric material used in the acoustic resonator.
What is needed, therefore, is a BAW resonator that overcomes at least the known shortcomings described above.